The present invention relates to wired and wireless IP telephone systems in a network having a dynamic address lease system.
From the purpose of cost reduction or application linkage, companies which shift a telephone system based on a conventional circuit switching network to an IP (Internet Protocol) telephone system operating on an IP network, are increasing in number. In the IP telephone system, a call control server is installed in a LAN (Local Area Network) formed in a company so that a user talks over using an IP telephone terminal. As a call control protocol between the call control server and the IP telephone terminal, SIP (Session Initiation Protocol) already standardized as RFC3261 “Session Initiation Protocol”, 2002, June (Literature 1) is well known. And most IP telephone systems in the future will be based on SIP as a mainstream. In this connection, the call control server based on SIP and the IP telephone terminal will be referred to merely as “the SIP server” and “the SIP terminal”, hereinafter.
Such a wireless IP telephone that a user can move with an IP telephone terminal carried by the user or can talk over while walking, receives much attention in these years, and a service to form an extension telephone system based on the wireless IP telephone in a company is provided by various communication service providers. In the extension telephone system using the wireless IP telephone, a wireless LAN is formed in the company and a user talks over using a wireless IP telephone terminal. Under the wireless IP telephone system, the user can send and receive a call anywhere so long as the user is in an available wireless LAN environment. Even when the user is in any subnet (which is a small network corresponding to one of divisions of the large network by network prefixes, terminals belonging to the same subnet having the same network prefix), the user can talk over. There also exists such a system that handoff can be realized, that is, a user can move from the current subnet to another subject while keeping his telephone talk.
How to realize a wireless IP telephone system based on SIP will be schematically explained. Since the SIP terminal is an IP terminal, an IP address is set manually or by such a dynamic technique as DHCP (Dynamic Host Configuration Protocol) (refer to RFC2131 “Dynamic Host Configuration Protocol”, 1997, March (Literature 2)). In a system using DHCP, a wireless IP telephone terminal has a DHCP client function. In other words, the wireless IP telephone terminal temporarily receives an IP address from the DHCP server installed in the LAN, and uses it, which action is also referred to “the DHCP server leasing the address to the DHCP client”. In the Literature 1, the SIP terminal (regardless of wired or wireless type) transmits a message called ‘REGISTER’ to a SIP registrar (which is a server for processing the REGISTER message from the SIP terminal). The REGISTER message contains a header called contact header, and the SIP terminal by utilizing the header informs the SIP registrar of the current IP address or FQDN (Fully Qualified Domain Name). The SIP registrar, when receiving the REGISTER message, informs a location server (which manages a database relating to positional information of the SIP terminal) of a correlation between the SIP URI (Uniform Resource Identifier) of the terminal as its sender and the IP address. And the location server in turn records the correlation between the SIP URI and the IP address. The correlation between the SIP URI and the IP address will be referred to as the SIP URI/IP address correspondence table, hereinafter. A series of operations from the transmission of the REGISTER message from the SIP terminal to the completion of registration to the location server will be called position registration, hereinafter.
Assume now that a SIP terminal (referred to as the communication party terminal, hereinafter) as the communication party made a call over the wireless IP telephone terminal after the position registration is completed. Calling in SIP is carried out by transmitting a message called ‘INVITE’. The INVITE message transmitted from the communication party terminal arrives at the SIP server managing the wireless IP telephone terminal. The SIP server transmits the INVITE message to the wireless IP telephone terminal by inquiring of the location server about the current IP address of the wireless IP telephone terminal. Accordingly, the INVITE message transmitted by the communication party terminal reaches the wireless IP telephone terminal, enabling the start of a speech. In place of transmitting the INVITE message from the SIP server, the SIP server may inform the communication party terminal of the current IP address of the wireless IP telephone terminal, the communication party terminal may retransmit the INVITE message to the current IP address. So far, the SIP registrar, the SIP server, and the location server have been explained to be located in different racks. However, all of these servers may be located in the same rack. These servers are assumed in the following explanation to be located in the same rack, and these servers will be collectively called the SIP server, hereinafter.
Explanation will be further made as to the operation when the wireless IP telephone terminal is shifted from the current subnet to a different subnet. These subnets will hereinafter be referred to as old and new subnets respectively. The wireless IP telephone terminal moves to the new subnet to establish a physical connection. This means, for example, to establish a connection with a access point in the new subnet, which is called establishment of an association. Thereafter, the wireless IP telephone terminal, using a dynamic address lease system such as DHCP, acquires an IP address to be used in the subnet, and transmits a REGISTER message to the SIP server as mentioned above to thereby perform position registration based on the new IP address. When a speech is being made between the communication party terminal and the wireless IP telephone terminal during movement between the subnets, the wireless IP telephone terminal transmits an INVITE message to the communication party terminal to inform the communication party terminal of the new IP address. Accordingly, the speech can be uninterruptedly continued even after the movement between the subnets.
There is also known such a service that the wireless IP telephone terminal utilizes the REGISTER message to be transmitted each time the terminal moves to another subnet to provide the position of the wireless IP telephone terminal as presence information. In this service, the SIP server or the presence server previously recognizes a correlation between the network prefix and physical position information (about company's conference room, living room, etc.), and thus can identify the position of the wireless IP telephone by collating it with the IP address of the wireless IP telephone terminal informed by the REGISTER message.